The coating of steel pipe used in the construction of pipe lines for carrying gas or oil may be carried out "over-the-ditch" or may be "plant-applied". Clearly, the major difference lies in the higher quality of the finished product made in plant and a major factor in this higher quality is in the higher quality of the cleaning and preparation of the pipe surface. It is, of course, also obvious that the in-plant equipment is generally more sophisticated and operational conditions more controllable leading to a better product as well. Theoretically, what can be done in-plant can also be done over-the-ditch, but, practically, this is not usually the case.
There are four major categories of presently used anti-corrosion external coatings for pipe-line pipe. These are
(1) Hot Applied coal-tar enamel and asphalt mastics in relatively thick layers (100 mils) and commonly reinforced on the outside with glass or asbestos sheets. While such coatings are reported to represent over half of the plant-applied coatings in the United States, the hazards presented by their use foretell a decreasing popularity of this category. Further, the products show poor impact resistance, poor resistance to mechanical penetration, poor abrasion resistance, poor stability to soil stress conditions, and are only deemed fair in regard to indentation resistance, pipe bending, cathodic disbonding and resistance to hydrocarbon solvents.
(2) Extrusion coatings of a thermoplastic resin: (typically 40 mils). In practice, polyethylene has virtually a monopoly in this area. The technique may involve a seamless tubular extrusion over the pipe or a flat die sheet extrusion wrapped over the pipe. In most cases, the polymer is applied to a first-applied mastic layer (e.g. bituminous). These coatings show improved properties in regard to those mentioned above for the Hot-Applied enamel and coal tar coatings except insofar as resistance to hydrocarbon solvents.
(3) Tape Coating System (typically 20 to 80 mils). This technique was developed, inter alia, to provide an improved and viable over-the-ditch method but it is, of course, a fine system for in-plant use as well. A corrosion protection type tape is spirally wrapped around a rubbed-based primer coating (previously applied) and a second plastic outer wrap tape may then applied. The tapes are generally polyethylene backings with a pressure-sensitive solvent, or primer-activated adhesive coating thereon. The properties exhibited by pipe coated by this system are similar to those of the extrusion coated pipes.
(4) Fusion-bonded coatings--A thermoplastic powder is electrostatically applied to hot pipe where it "melts", adheres to the metal and fuses to itself. Only three basic materials have been widely used--polyethylene, vinyl and epoxy powders with only the latter enjoying commercial success in the United States. Chemically, these are, generally, bis-phenol A polymers with epoxy end groups.
The epoxys require a thermal curing to the thermoset form and usually a catalyst is used in the system (e.g. amines, acids, boron halides, etc.). Often times a liquid epoxy primer is used prior to the powder coating. Typically epoxy coatings have been 12-14 mils in thickness to provide at least acceptable resistance to cathodic disbonding, although in M. D. Simpson's paper "External Protection of Steel Pipes Using Epoxy Powder Coatings" (contribution SI) presented at the Second International Conference on the Internal and External Protection of Pipes (in England Sept. 1977) he states (page X2)
"Bitumen coal tar and polyethylene are required to be applied relatively thick, but epoxy powder coatings give excellent protection with only 3 mm of coating."
Apparently and obviously "3 mm" should be --0.3 MM-- (12 mils) nevertheless, this value (i.e. 0.3 MM) still represents a relatively thick coating and its attendant disadvantages, e.g. brittleness and lack of flexibility and stresses at the pipe-epoxy interface.
In order to effect a satisfactory epoxy coating which would have satisfactory resistance to impact, mechanical penetration, indentation, abrasion, soil stresses, and cathodic disbondment it has been accepted that about a 12-14 mil thickness coating is required.
Some patents relating to the application of wrapping of thermoplastic material on pipe are U.S. Pat. Nos. 3,616,006 to Landgrof; 3,687,765 to MacLean et al; 3,802,908 to Emmons and 3,823,045 to Hielema. Extrusion coating of pipe is taught in Colombo U.S. Pat. No. 2,820,249. Epoxy coatings are described in U.S. Pat. Nos. 4,060,655; 3,578,615; 3,508,946; and 3,258,032. All of these patents are incorporated herein by reference thereto.